Field of the Invention
The invention concerns a method for a rapid determination of a spatially resolved magnetic resonance relaxation parameter in one area of examination, such as the longitudinal relaxation constants T1 and the transverse relaxation constants T2.
Description of the Prior Art
Medical magnetic resonance imaging is useful in the diagnosis of soft tissue due to the high contrast especially for imaging of different tissues or for the visual representation of the state of the tissue. The spatially resolved determination of the value of magnetic resonance relaxation parameters, such as the longitudinal relaxation constant T1 or the transverse relaxation constant T2, has also an added importance in medical magnetic resonance technique, because it allows to diagnose different tissues or to identify abnormal tissue changes. The spatially resolved and quantified relaxation parameters are represented in the form of parameter maps and are thus a useful demonstration tool in the diagnosis.
Particularly in the diagnosis of diseases of the heart tissue, quantitative methods for the determination of the relaxation parameter have increasingly gained in importance. Of importance in this context is the quantitative determination of the longitudinal relaxation constant T1.
However, the determination of the T1 constant of the heart tissue is a major challenge due to cardiac and respiratory movement. This is even more difficult for long T1 relaxation constants, as they typically occur in the cardiac tissue before the administration of a contrast agent. Depending on the data acquisition technique, irregular heart and respiratory cycles represent a further complication in determining the T1 constant. This is so because, for a precise quantification of T1 constants, we must know the longitudinal magnetization. If the repetition time is too short, before a new excitation, the longitudinal magnetization has not completely returned to its original position. The effective repetition time, which with heart rate-triggered data recording is determined by the heart rate, must then be taken into account.
Known methods for T1 quantification in cardiac tissue detect the magnetic resonance data in a breath hold phase over several cardiac cycles. Here, several inversion pulses triggered are irradiated into the imaged area, which is then followed by an acquisition phase and a recovery phase of the longitudinal magnetization.
The publication of Daniel R. Messroghli et al. “Modified Look-Locker Inversion Recovery (MOLLI) for High-Resolution T1 Mapping of the Heart”, published in Magnetic Resonance in Medicine 52:141-146 (2004), describes a process in which a special interlocking nested acquisition scheme is applied to obtain magnetic resonance data with different inversion times. The acquisition schedule is triggered by the heart rate so that the data can always be obtained from the same heart phase, i.e., from the same state of motion of the heart.
While the special data acquisition scheme in MOLLI can, to a large degree, avoid motion artifacts arising due to the cardiac motion, there can still arise motion artifacts if the patient, who is being examined during the data acquisition, does not completely stop breathing. The motion artifacts prevent an accurate determination of the T1 constants, because at least two measurements must be taken from the same voxel (as the image data “pixel”) at different times during the relaxation.
A method for motion correction of MOLLI-acquired data for T1 mapping of cardiac tissue is described in Hui Xue et al. in the publication “Motion Correction for Myocardial T1 Mapping Using Image Registration with Synthetic Image Estimation”, published online in Magnetic Resonance in Medicine on 29 Aug. 2011. The described motion correction is based on the estimate of synthetic images that have a similar contrast as the originally acquired images. Here, the image contrast, which changes as a result of the relaxation and the movement, is formulated as a joint estimation problem. Using an iterative process and an initial estimate of T1 (T1 start value), a sequence of movement-corrected synthetic inversion recovery (IR) images are generated. With the initial estimate of T1, synthetic images are calculated for each inversion time. The synthetic images are then created all at a particular breathing position and have a similar contrast as the originally measured images. They are therefore suitable for an intensity-based image registration. This method is described for the correction of respiratory motion.
The known methods for spatially resolved determination of relaxation parameters of a beating heart are performed over several heartbeats, and thus require a relatively long measurement time.